After protesting the budget Suffolk County leaders approved last fall, environmental groups have now sued the county over its use of nearly $33 million in funds that were raised through the Drinking Water Protection Program, a self-imposed tax that Suffolk residents have voted to levy upon themselves several times since the late 1980s.

Saying that the funds comprise one of several dedicated revenue streams created by the sales tax — which will be in effect until 2030 — critics claim the choice to use it to close a budget gap violates the terms under which voters agreed to tax themselves.

“What Suffolk politicians did was not just illegal, it was a violation of the public trust,” said Richard Amper, executive director of the Long Island Pine Barrens Society, which brought the suit together with the Long Island Environmental Voters Forum. “Citizens and taxpayers voted to give government more than $1.5 billion to protect water with the assurance that the funds could not be used for any other purpose without another vote by the people.”

The Drinking Water Protection Fund is filled through a sales tax of one-quarter of one percent. Within that revenue stream are several specific uses, such as open space purchases and a fund dedicated to stabilizing sewer rates for residents. The 2014 budget used $32.8 million from the county’s sewer stabilization fund.

The lawsuit demands that the county return the money to the sewer stabilization fund, along with interest.

Justin Meyers, communications director for Mr. Bellone, said on Monday afternoon that the county exec had met with Mr. Amper and others who had brought the suit, describing the meeting as friendly.

“The fact of the matter is that there are two overarching concerns,” he said. “First, if the money is being taken and used for something other than drinking water, it must be repaid. The county executive completely supports that.”

He added that also, the county “needs to engage the public and voters on the issue if it moves forward.”

Mr. Meyers added that once the county decides to spend the money from the sewer stabilization fund, the county legislature would have to pass a measure approving the spending. Within the language of that approval would be a repayment structure outlining when the county would pay the fund back.

“Our argument has been that money is in this account, just sitting there,” he said. “So this is a way to save taxpayers money, instead of bonding and borrowing.”

Suffolk voters last agreed to renew the tax in 2007 — approving a ballot measure to maintain the tax through 2030. The recent plan laid out by the county intends to start paying back into the sewer stabilization fund in 2017. Last fall, the balance hovered around $140 million, leaving over $100 million left in the sewer stabilization fund.

The lawsuit was filed on Wednesday, and names Suffolk County Executive Steve Bellone, the Suffolk County Legislature and the County of Suffolk all as defendants.

Three legislators voted against the budget last fall: Legislator Tom Barraga (R-East Islip), Jay Schneiderman (I-Montauk) and Tom Cilmi (R-Bay Shore). Mr. Bellone’s original budget had not called for dipping into the sewer stabilization fund at all, but rather closing the budget gap in the $2.7 billion budget through borrowing from the New York State Dormitory Authority, a path that would have required legislation approved at the state level. A report from the County’s Budget Review Office identified that plan as a risk because of the necessary legislation.

An opinion of the county attorney’s office issued last fall defended the use of the funds.

Provided by a spokesperson for Mr. Bellone, the county pointed to case law — considered analogous with Suffolk County — that held that “The New York Court of Appeals has endorsed the statement that ‘laws proposed and enacted by the people under an initiative provision are subject to the same constitutional, statutory, and charter limitations as those passed by the legislature and are entitled to no greater sanctity or dignity.’”